This invention generally relates to a venous and cardiotomy reservoir. More particularly, the invention relates to a venous and cardiotomy reservoir which incorporates an outlet reservoir that exhibits an enhanced sensitivity to the level or volume of blood in the reservoir outlet. The reservoir is particularly adapted for use with a collapsible peristaltic pump.
Reservoirs and pumps of the above general type have found particular utility in various medical fields for transferring bodily fluids and blood between a patient and one or more extracorporeal devices. Medical procedures which commonly employ peristaltic pumps and reservoirs include, without limitation, open heart surgery for circulating blood between a patient and a heart lung machine, dialysis procedures for transferring blood between a patient and dialyzer, and cardiopulmonary bypass (CPB) surgery where blood is removed from the open thoracic cavity, oxygenated and returned to the patient.
A collapsible peristaltic pump can also be used to advantageously control the liquid level in a reservoir upstream from the pump. This is achieved by strategically positioning the pump inlet at a height equal to the lowest allowable liquid level in the reservoir. Taking advantage of the hydrostatic pressure of the liquid in the reservoir, when the liquid level in the reservoir drops to the height equivalent of the pump inlet, the pump tubing will collapse and no further pumping will take place, even though the pumping mechanism itself may continue to operate.
During CPB surgery, blood is typically drawn from a venous blood vessel through a venous catheter into a venous reservoir. The blood is then pumped through an oxygenator, heater or other extracorporeal device and returned through an arterial catheter to an arterial blood vessel of the patient. The reservoir is often an open reservoir located between the patient and the inlet of the pump. An open reservoir is formed from a rigid container that is vented to ambient air. When blood enters the open reservoir it displaces a portion of the air in the reservoir while maintaining an air to blood interface. When provided in the CPB surgery blood circuit, the reservoir allows, among other things, storage of blood, elimination of air bubbles within the blood, filtering of the blood, defoaming of the blood, and controlling the amount of suction applied to the venous catheter. The amount of suction is typically limited by gently siphoning the venous blood via gravity into the open reservoir.
A potential hazard during CPB surgery is the emptying of the blood from the venous reservoir and the pumping of air into the patient. Emptying of the reservoir could occur if the flow rate out of the reservoir is higher than the venous drainage from the patient into the reservoir. Although the rate of venous drainage is not measured, it is the responsibility of the perfusionist (a technician trained to operate a heart/lung machine) to maintain a safe level of blood in the reservoir (usually 300 to 500 ml). The perfusionist must therefore adjust the pump speed to accommodate changes in the venous supply. Despite close control, the reservoir can quickly be emptied while the perfusionist is temporarily distracted and, consequently, air can be pumped into the patient. While electronic level sensors are commonly used to warn the perfusionist of a low blood level in the reservoir, these sensors can malfunction and fail.
Peristaltic pumps are volumetric pumps in which a rotating or linearly moving member uses rollers at spaced apart intervals to progressively compress a flexible tube and propel a fluid (e.g. blood) through the tube. The principal advantage of the peristaltic pump is its simplicity of operation and an absence of contact between the fluid and frictional surfaces, including valves, which can be responsible for a variety of hazards such as the hemolyzing of blood cells. Thus, instead of directly contacting the rotating member of the pump, the fluid only contacts the chemically inert tube. Another type of peristaltic pump is a collapsible peristaltic pump.
Collapsible peristaltic pumps utilize a tube which becomes completely occluded in its free condition (when the pressure within the tubing is equal to the pressure surrounding the outside of the tubing). Collapsible peristaltic pumps are described in U.S. Pat. Nos. 5,222,880 and 5,281,112, which are herein incorporated by reference. These types of pumps are advantageous because they inherently regulate output flow. Specifically, the output flow of the pump is dependent on the inlet pressure of the fluid into the pump. As a result of this feature, if the line upstream of the pump becomes occluded, emptied or otherwise fails to supply fluid to the pump, the tube of the pump will occlude and preventing the pump from generating dangerously low negative pressures. Such pressures can significantly hemolyze the blood and can empty the tissue vessel of the patient resulting in a collapse of the vessel and damage to the tissue itself.
With the above limitations in mind, it is an object of the present invention to provide a venous blood reservoir which can be used with a variety of peristaltic pumps including collapsible peristaltic pumps and which can be used in conjunction with a cardiotomy reservoir.
It is also an object of this invention to provide a reservoir which substantially eliminates the need for close control by a perfusionist over the blood level.
Another object of this invention is to provide a reservoir having a distinct outlet portion which has a greater sensitivity to changes in the level or volume of blood in the reservoir.
Yet another object of this invention is to provide a reservoir which incorporates a safety level defined as the level below which the liquid in the reservoir should not be allowed to go.
Another object of this invention is to provide a reservoir in which the level sensitive outlet portion of the reservoir is capable of containing a volume which is at least equivalent to the stroke volume of the pump with which it is used.
A further object of this invention is to provide a reservoir having an outlet portion whose height or level, when containing maximum volume, will provide a hydrostatic pressure sufficient to ensure at least a 50% maximum output by the pump.
Still another object of this invention is to provide a blood pumping system which incorporates a reservoir having a outlet portion of enhanced level sensitivity and used in conjunction with a collapsible peristaltic pump.
A further object of this invention is to provide a reservoir which includes a mechanism for preventing the outflow of blood from the reservoir when the blood level in the reservoir falls below a minimum acceptable level.
In achieving the above and other objects, the present invention provides for a venous blood reservoir which is defined as having a primary or upper portion or reservoir that smoothly transitions into an outlet or lower portion or reservoir. As suggested, the outlet reservoir is located beneath the primary reservoir. The primary reservoir defines an upper cavity and may include inlet ports which permit the inflow of blood into the reservoir. The blood is disposed into the reservoir at a location beneath the blood/air interface to prevent splattering of the blood and bubble formation. The outlet reservoir defines a lower cavity and includes an outlet port which permits the outflow of blood from the reservoir to the pump. The outlet reservoir has an average cross sectional flow area that is less than the average cross sectional flow area of the primary reservoir. Because of this difference in cross sectional flow area, the outlet reservoir has an increased sensitivity in exhibiting, in terms of blood level, changes in the blood volume. A perfusionist is therefore able to more readily detect the rate of change of the blood level in the outlet reservoir as compared to the rate of change of the blood level in prior reservoirs or even the primary reservoir of the present invention.
A safety level is also defined in the outlet reservoir. As further discussed below, this safety level defines that portion of the outlet reservoir which is to correspond in height to the inlet of the collapsible peristaltic pump. The volume of the outlet reservoir above the safety level is equal to at least one stroke volume of the pump to provide the perfusionist with a factor of safety as well as a visually determinable gauge as to the specific amount of blood left in the outlet reservoir. Since the output flow of a collapsible pump is dependent on the inlet pressure of the fluid into the pump, the highest level of the blood in the outlet reservoir is an amount sufficient to create a hydrostatic pressure that will generate at least a 50% maximum output of the pump. Thus when the level of blood is within the outlet reservoir, at or above the safety level, the perfusionist is therefore aware that the pump output flow may be seriously compromised.
The outlet of the present reservoir is connected by a conduit to the inlet of a collapsible peristaltic pump. As suggested above, the height of the inlet is set to correspond with the height of the safety level defined in the outlet reservoir, or vise versa. In this manner, when the level of blood in the outlet reservoir reaches the safety level, the pressure on the interior and exterior of the collapsible tube of the pump will be substantially equal causing the tube to occlude. This occlusion prevents the generation of negative pressures within the pump and the pumping of air into the patient. This occurs regardless of whether the transfer mechanism of the pump continues to operate or whether it is stopped.
In an alterative embodiment, the present invention includes a mechanism which operates in response to the level of blood in the outflow reservoir to prevent the outflow of blood from the reservoir once the level of blood therein reaches a critical or safety level. While in the prior embodiment this is achieved by a height adjustment of the outlet reservoir safety level relative to the inlet of the collapsible peristaltic pump, in the second embodiment a check valve or other mechanism is incorporated into the outlet reservoir. This enables the reservoir to be used not only with collapsible peristaltic pumps, but also with other pumps that do not generate significant negative pressures at their inlets.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.